This invention relates to a novel bag making machine for producing thermoplastic resin film bags. More specifically, this invention relates to a machine for making bags and a control system which includes a timer for operably controlling production of bags made from thermoplastic resin films of varying thickness.
Thermoplastic resin bags, such as those used in supermarkets, convenience stores, etc. are produced from a long tubular web of thermoplastic synthetic resin film. The tubular web, which is unrolled from a spool, is intermittently fed into a bag making machine where heat is transversely applied to the web in order to form a weld seal between the two layers of the web. A cutting edge intersects and transversely severs the web adjacent to the seal in order to complete formation of a bag.
A common configuration for achieving the above mentioned sequence of operations includes platen rollers which intermittently draw a web of thermoplastic resin film into the bag making machine. Upon cessation of advancement of the web, a sealing mechanism, which includes a fixed lower seal surface and a heated upper reciprocating seal bar, engage and melt the top and bottom layers of the web together. A weld bead seal is formed when the web cools.
The sealing surfaces are coated with an anti-adhesive material, such as Teflon, in order to prevent the weld bead seal of the thermoplastic web from adhering to the seal bars. Associated with the upper reciprocating seal bar is a cutting edge which transversely severs the thermoplastic web adjacent to the weld bead. Platen rollers then advance the web the length of a bag in order to repeat the sealing and severing cycle.
There are various desirable configurations of bags and an accompanying variety in the thickness of the thermoplastic resin film which forms a bag. The thickness of the thermoplastic resin films may range between 0.01 and 0.2 millimeters. This relatively wide range of thicknesses necessitates differing time periods during which the sealing bar melts the bag so that a weld bead of high structural seal integrity may be formed.
One previously known bag making machine assembly includes a single motor which drives both the sealing and severing means and the platen rollers, which advance the web. The constant speed of rotation of the motor alternately drives the advancement rollers and the heat sealing and cutting mechanism. A clutch, brake, and crank assembly effect the alternating operation of the advancement and sealing and severing operations. In order to lengthen the amount of time that the sealing bar applies heat to the web, a cam connected to the assembly can be adjusted, thereby altering the duration of the sealing operation. However, adjustment of the cam assembly is time consuming and an ineffective method of varying seal time. Increasing the sealing time requires that the speed of rotation of the motor is slowed. As a result, the platen rollers, which are also responsive to this single motor, advance the web at a slower rate, thus decreasing machine efficiency. The speed of rotation of the motor limits the rate of advancement of the thermoplastic resin film, thereby adversely affecting overall machine efficiency.
The difficulties suggested in the preceeding are not intended to be exhaustive but rather are among many which may tend to reduce the efficiency of bag making machines when presented with varying thicknesses of thermoplastic resin film. Other noteworthy problems may also exist; however, those presented above should be sufficient to demonstrate that bag making machines appearing in the past will admit to worthwhile improvement.